A numerical study of picosecond laser micro-grooving of single crystalline germanium: Mechanism discussion and process simulation

Abstract

The temperature field evolution and material removal process during picosecond (ps) laser micro-grooving of single crystalline germanium (Ge) are explored by developing a 2D two-temperature model (TTM). The transient temperature-dependent thermal and optical properties of electrons and lattice are incorporated for accurate calculations, and the electron-lattice coupling strength is estimated considering the electron transition from valence band (VB) to conduction band (CB) mainly caused by one photon absorption. Different material removal mechanisms, i.e. the thermal way caused by vaporization and the athermal manner resulted from covalent bonds breakage, are proposed and compared with experimental results. The variation trends of the thermal model predicted groove depths are similar and consistent with the measured ones within selected ranges, while the athermal model leads to groove depth far from reality, demonstrating that the vaporization-caused mechanism is the proper one. Employing the verified model, simulation study is conducted to explore the evolution of temperature field under different fluences. Under a low fluence of 0.071 J/cm 2 , the peak electron temperature shows a decreasing trend with pulse width, and the temperatures of electron and lattice evolve in a similar pattern when the pulse duration is wider than 200 ps. For a higher fluence of 7.074 J/cm 2 , lattice temperature increases up to melting and boiling point within 1 ps and 3 ps, respectively, and the groove depth produced by a 12 ps pulse heating is about 0.65 μm. Afterwards, residual heat diffusion during pulse interval and further material removal during following pulse heating will occur, leading to a deeper groove. Moreover, the effects of laser power and critical free electron density on the machining process are also analysed, demonstrating that an increase in any of these two parameters results in a faster element removal. • Free electron density variance in Ge under laser irradiation is considered. • Comprehensive temperature-dependent properties are incorporated. • The thermal coupling strength depends on temperature and free electron density. • Material removal is confirmed to be a thermal process via model verification. • Shorter pulse width leads to larger electron-lattice temperature gap. • The groove formation process under high fluence is detailed simulated.